A salvo is when troops fire their guns all at the same time. A salvo of shots might signal the beginning of a military battle.
Another word for the gunfire kind of salvo is a "volley." Military planners have used salvos as a way to overwhelm an opposing army, including during engagements between battleships and tanks. You can also use salvo in a more figurative way, describing a verbal barrage or attack: "Her direct, almost rude questions were seen as the opening salvo in the battle between the two political candidates."